This invention relates to an electric motor with permanent-magnet excitation, having motor pans which are movable relative to one another, of which one motor pan forms a multi-pole excitation field in an air gap by means of permanent magnets and the other motor part comprises a coil configuration situated in this air gap, the permanent-magnetically excited motor pan comprising a cylindrical yoke having substantially radial slots, which are uniformly spaced about the circumference and in which permanent magnets are mounted, which permanent magnets are magnetised in a circumferential direction with directions of magnetisation which change from magnet to magnet.
The generation of multi-pole magnet fields is one of the most important technical uses of permanent magnets, most of these components being used in permanent-magnet motors. These components may have different shapes. The most important shapes include rotationally symmetrical disks, which produce a multi-pole magnetic field in an axial direction, rectangular blocks, which generate a multi-pole field directed perpendicularly to one of the block surfaces, and cylinders or hollow cylinders, which generate a multi-pole magnetic field in a radial direction.
DE 36 07 648 C2, which corresponds to U.S. Pat. No. 4,679,313 Jul. 14, 1987 discloses a motor in which permanent-magnet annular segments are arranged on an iron sleeve, thus forming a cylindrical permanent-magnet rotor. The permanent magnets are magnetised in such a manner that in the direction of the air gap each north pole is followed by a south pole. The rotor is surrounded by a cylindrical stator without winding slots. The winding is arranged in the air gap between the permanent-magnet rotor and a soft-magnetic stator sleeve.
If only those materials which are currently used in large quantities are considered, these segments can be made either of ferrites, which are cheap but which generate comparatively weak magnetic fields, or rare-earth materials, which are expensive but which produce strong magnetic fields.
For both groups of materials a distinction is made between sintered and resin-bonded materials. The sintered materials are anisotropic, as a result of which the magnetic fields produced by them are stronger than those produced by the isotropic resin-bonded materials. The disadvantage of sintered materials is that they can be manufactured only in simple shapes and these shapes are anisotropic only in a few directions which are dictated by physical properties. Therefore, only simple magnetisation patterns can be produced. Moreover, it is very difficult to manufacture sintered magnets with narrow tolerances of their mechanical dimensions. Conversely, resin-bonded materials have the advantage that components with complicated geometries and narrow mechanical tolerances can be manufactured at low cost and that by means of a suitable magnetisation process it is also possible to obtain intricate magnetisation patterns in the material.
A problem when generating multi-pole radial magnetic fields is that to date cylinders or hollow cylinders with radial anisotropy cannot be manufactured in an industrial process. It is possible to manufacture cylinders or hollow cylinders with diametral anisotropy, but these are only capable of generating two-pole magnetic fields and are therefore not suitable for uses requiring magnetic fields with a larger number of poles.
The manufacture of the rotors described in DE 36 07 648 C2 is intricate because the permanent-magnetic annular segments each have to be glued onto the rotor sleeve. To guarantee a reliable operation rotor binding is necessary. Large magnetic fields can be obtained when-the magnet segments are made of an anisotropic material. However, even the production of annular segments from an anisotropic material is already very intricate.
From DE 42 16 938 (PHD 92.057) a permanent-magnet rotor is known which consists of an iron yoke having radial slots in which block-shaped permanent magnets are mounted. These permanent magnets are magnetised circumferentially and have a direction of magnetisation which changes from magnet to magnet in the circumferential direction. These rotors can be manufactured more simply than the rotors known from DE 36 07 648 C2. Gluing the blocks into the slots is not a problem. No additional fastening is required. Their simple geometry allows the block-shaped magnets to be simply made of an anisotropic material and to be subsequently magnetised so that large magnetic fields can be produced.